Obstructive sleep apnea is an airway breathing disorder caused by relaxation of the muscles of the upper airway to the point where the upper airway collapses or becomes obstructed by these same muscles. It is known that obstructive sleep apnea can be treated through the application of pressurized air to the nasal passages of a patient. The application of pressurized air forms a pneumatic splint in the upper airway of the patient thereby preventing the collapse or obstruction thereof.
Within the treatment of obstructive sleep apnea, there are several known CPAP regimens including, for example, mono-level CPAP and bi-level CPAP. Mono-level CPAP involves the constant application of a single therapeutic CPAP level. That is, through the entire breathing cycle, a single therapeutic positive air pressure is delivered to the patient. While such a regimen is successful in treating obstructive sleep apnea, some patients experience discomfort when exhaling because of the level of positive air pressure being delivered to their airways during exhalation.
In response to this discomfort, bi-level CPAP regimens were developed. Bi-level CPAP involves delivering a higher therapeutic CPAP during inhalation and a lower therapeutic CPAP during exhalation. The higher therapeutic CPAP level is commonly known as inspiratory positive airway pressure or “IPAP.” The lower therapeutic CPAP level is commonly known as expiratory positive airway pressure or “EPAP.” Since the EPAP is lower than the IPAP, the patient needs to do less work during exhalation to exhale and thus experiences less discomfort, compared to the mono-level CPAP regimen.
However, the development of bi-level CPAP significantly increased the sophistication of CPAP devices because the devices must accurately determine when the patient is inhaling and exhaling and to properly coordinate the IPAP and EPAP levels thereto. One approach is to determine the instantaneous and average flow rates of air being delivered to the patient and then to compare the two to determine whether a patient was inhaling or exhaling. If the instantaneous flow rate is greater than the average flow rate, the patient is deemed to be inhaling. If the instantaneous flow rate is less than the average flow rate, the patient is deemed to be exhaling. However, using the instantaneous and average flow rates of the air being delivered to the patient has several disadvantages including accuracy and response time. In this regard, the flow of air is generally turbulent and therefore difficult to measure accurately. Additionally, leakages caused by loose fitting patient breathing interfaces such as, for example, nasal and mouth masks, contribute to the difficulty of determining accurate air flow rates. Closely connected thereto, the turbulent flow and difficulty of accurate measurement necessarily cause a slow response time in changing between IPAP and EPAP levels. Hence, a bi-level CPAP device that does not suffer from these deficiencies is highly desirable.